Dutch brewing giant Grolsch has launched an iPhone app to help drinkers test out how inebriated they are by challenging their ability to walk in a straight line.

The "Walk the Line" app was launched at the North Sea Jazz Festival in the Netherlands. Users who got a high score were offered the chance to get a free festival photograph taken at the Grolsch bar, but those with a low score were invited to join the Grolsch team for a drink of water.

While the app is not actually an alcohol test, Grolsch says it aims to encourage festival-goers to monitor their alcohol consumption in a "fun, innovative way" and share it with their friends.

Grolsch is taking the concept to other major music festivals this summer, with promotions staff encouraging participants to download and play with the app.

Marvellous, Amsterdam, part of Aegis Group's Isobar network, created the app using the iPhone's motion-sensing accelerometer. It is also available in a generic version. But it's not available in the U.S.